I didn't see this when it came in, so it's a little stale. I also can't answer the question exactly. But I can tell you about an experience I had.

We use an RFS4000 with around 100APs. Our APs are set to bridge -- not tunnel -- traffic, meaning the RFS4000 itself is not involved in handling wireless traffic; it only manages the APs. We had an issue on the controller where the temperature sensor failed. The hardware was otherwise provably good, but the OS refused to boot, because it thought the system was overheated. We didn't have a spare at the time, and it took about 4 days to acquire and provision a replacement. Wireless service during this time was... not good. It was surprisingly very bad. This is in spite of using a controller that should never process any traffic on it's own.

The point is you almost certainly do need to keep the licenses available for all of your APs. Removing the licenses will limit the controller from managing all of them, and the controllers ability to manage the APs absolutely does make a HUGE difference.